What’s a conduction band?

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Used in quantum mechanics, the term conduction band refers to a combined orbital area, or band, for electrons in a molecule. Unlike the valence band, the conduction band rarely contains electrons. In excited states, electrons will move momentarily into the conduction band before releasing their energy and falling back into the lower electron orbitals. Understanding the behavior of electrons with respect to this band is useful for understanding the behavior of various substances. In quantum mechanics, the concept of conduction band is addressed in band theory.

Atoms are organized with protons – positively charged particles – and neutrons – neutral particles – clustered in the center. The electrons, tiny negatively charged molecules, orbit the central cluster, similar to the way the planets of the solar system orbit the sun. Like planets, electrons have fixed orbits. Unlike planets, however, electrons can move in a different orbit if they gain enough energy.

Generally, electrons are found in the lower orbitals of an atom. Electrons will always fill the lowest orbital first, only moving on to the next when the first is full. This natural position is called the ground state of the atom.
An atom’s valence electrons, or those usually found in the outermost band of ground-state orbitals, can be shared with other atoms. In covalent bonds, the valence electrons of multiple atoms share their orbitals. The original orbitals of the valence electrons get confused, creating a valence band in the molecule.

When electrons gain energy, or reach an excited state, they can move to higher orbitals, which are in the conduction band. The electrons must have enough energy to jump over a non-electronic area, or band gap, to reach the conduction band. Since electrons ultimately prefer to be in the ground state, once in the conduction band, they release energy in the form of photons of light and fall back into their valence band orbitals. The total time an electron spends in the conduction band is less than one second.

The ability of electrons to reach the conduction band determines the electrical conductivity of an object. Different substances have different band gap sizes, so some substances require less energy to move electrons between orbitals. For example, conductors have a small band gap, so electrons don’t require much energy to jump this tiny gap and reach the conduction band. That’s why conductors are ideal for conducting electricity. Conversely, insulators have a very large band gap, so they require much more energy for the electrons to make the jump, and therefore don’t conduct electricity well.

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